Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

Martyna Cybularczyk-Cecotka, Anna Dąbrowska, Piotr Guńka, Paweł Horeglad
2018 Inorganics  
The investigation of the reactivity of six membered N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes) towards dialkylgallium and dialkylindium alkoxides/aryloxides has shown that both steric hindrances and donor properties of 6-Mes significantly influence the strength of M-C 6-Mes bond, as well as the formation, structure and reactivity of Me 2 MOR(6-Mes) (M = Ga, In) complexes. While the reactions of simple dimethylgallium alkoxides with 6-Mes
more » ... ead to the formation of stable monomeric Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1) and Me 2 GaOMe(6-Mes) complexes, the analogous Me 2 InOR(6-Mes) are unstable and disproportionate to methylindium alkoxides and Me 3 In(6-Mes) (2). The use of bulky alkoxide ligand-OCPh 2 Me or aryloxide ligand-OC 6 H 4 OMe allowed for the synthesis of stable Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga (3) and In (4)) as well as Me 2 M(OC 6 H 4 OMe)(6-Mes) (M = Ga (5) and In (6) ). The structures of 1-6 have been determined using both spectroscopic methods in solution and X-ray diffraction studies, which confirmed the effect of both steric hindrances and donor properties of 6-Mes on their structure and catalytic properties in the ring-opening polymerization (ROP) of rac-lactide. Inorganics 2018, 6, 28 2 of 16 M-O alkoxide bond. On the other hand, rac-LA could insert, even at low temperature, into In-C NHC bond of Me 2 GaOR(NHC), considerably weaker in comparison with Ga-C NHC [3]. Analogously, the insertion of lactide into the M-C NHC bond of Mg alkoxides with alkoxide ligands possessing NHC termini [16], Me 3 Al(NHC) adducts [21], or yttrium and titanium complexes [22], have been reported so far. Similarly to the observed effect of a metal on the corresponding M-C NHC bond, as well as the structure and ROP activity of discussed main group metal alkoxides with NHCs, we should expect a considerable effect of steric and electronic properties of NHC on both the strength and reactivity of the M-C NHC bond. Our initial studies revealed the adverse effect of steric hindrances of NHC on the synthesis of Me 2 GaOR(SIPr) as well as the formation and strength of Ga-C SIPr bond [2], which was additionally supported by the structure of Me 3 M(NHC) (M = Al, Ga, In) adducts [3, 21, 23] . However, the more detailed effect of both steric and electronic effect of NHC on the M-C NHC bond has not been addressed so far for main group metal alkoxides with NHCs, including Me 2 MOR(NHC) (M = Ga, In) complexes. In order to probe the effect of NHC on the latter, we chose 6-Mes-an N-heterocyclic carbene of stronger donor properties, along with the increased steric demand, in comparison with SIMes or IMes (Figure 1) [24, 25] . While the increased steric hindrances of 6-Mes result from wider N-C-N angle, this should be responsible for the hybridization at carbene carbon and the higher energy of HOMO orbital of 6-Mes occupied by a lone pair of electrons, and therefore its donating properties [26] .
doi:10.3390/inorganics6010028 fatcat:zfftjfoivjfkhnqxchvqryzkny